Hydrogen sulfide and mercaptans are toxic, corrosive and malodorous compounds. Each may be found in a variety of liquid and gaseous media such as natural gas, petroleum, refinery gas streams, carbon dioxide, hydrogen, coal gas streams, gas streams from viscose rayon production, tars and asphalt, shale gas, coke oven gases, ammonia synthesis gas, rubber vulcanization streams, gases from sulfurization plants, turpentine production, pulp and paper mill effluent, sewage, brine drilling mud, land fills, phosphoric acid production gas streams and other industrial gas streams and effluents. They are also found in the tail gases and liquids of some hydrogen sulfide scrubbing processes such as Claus plants and amine scrubbing units. Such sulfur compounds include hydrogen sulfide, allyl thiol, propane thiol, benzyl thiol, crotyl thiol, ethane thiol, thiocresol and thiophenol.
Hydrogen sulfide is an undesirable contaminant which is highly toxic, corrosive and has an objectionable odor. The release of hydrogen sulfide into the environment is strictly controlled by the Environmental Protection Agency and Department of Environmental Resources, as well as by other regulatory agencies throughout the world. Hydrogen sulfide and mercaptans not only have an offensive odor, but have also been linked to the formation of acid rain.
Methods for removing hydrogen sulfide and/or mercaptans may be generally classified as regenerative and non-regenerative. Regenerative processes are generally more desirable because waste products are recycled. By regenerating sulfur scavenging compounds and thereby recycling the waste products, the cost, both economically and environmentally, of replenishing spent chemicals in the process and disposing of the waste products is lessened or eliminated. However, typical prior art regenerative processes often require expensive processing equipment and high energy input.
Various amines and alkanomines, which may be regenerated, have been used to remove acids, such as hydrogen sulfide, from gas streams. U.S. Pat. No. 2,776,870 discloses that aqueous amines and alkanolamines are useful for removing acids from a gaseous mixture. Hydrogen sulfide may be selectively removed from gas streams containing carbon dioxide by use of triethanolamine or methyldiethanolamine.
British Published Patent Specification No. 2103645 discloses that hydrogen sulfide and carbon dioxide may be removed from a gas mixture by contacting the mixture with a solvent comprising a tertiary amine and a physical absorbent. Suitable physical absorbents include N-methylpyrrolidone and sulfolane.
U.S. Pat. No. 4,112,051 discloses a process for removing acidic gases from a gaseous mixture with an amine-solvent liquid absorbent comprising (1) an amine comprised of at least about 50 mole percent of a sterically hindered amine; and (2) a solvent for the amine mixture which is also a physical absorbent for the acidic gases. Suitable sterically hindered amines include various piperidine compounds. Suitable solvents include sulfones and pyrrolidone and piperidone compounds, to name a few.
U.S. Pat. No. 4,978,512 discloses methods for reducing the levels of hydrogen sulfide and organic sulfides in a hydrocarbon stream by contacting the stream with a composition comprising a reaction product of a lower alkanolamine with a lower aldehyde. Suitable reaction products include mixtures of triazine and bisoxazolidine compounds.
U.S. Pat. No. 4,647,397 discloses a process and composition for removing hydrogen sulfide and similar sulfides from a gas stream. The gas stream is contacted with a substituted aromatic nitrile having an electron-attracting substitute on the aromatic ring at least as strong as halogen and an organic tertiary amine in an inert organic solvent, such as N-methyl-2-pyrrolidone. The spent contacting solution may be regenerated by heating the solution above the decomposition temperature of the reaction products to separate the sulfides from the liquid phase absorbent solution.
U.S. Pat. No. 4,775,519 discloses a continuous process for removing acid gas components from a gas stream by countercurrently contacting the stream with an aqueous solution of a mixture of N-methyldiethanolamine (MDEA) with imidazole or a methyl substituted imidazole. The gas is deabsorbed from the MDEA and the imidazole by reducing the pressure and causing the gas to flash.
U.S. Pat. No. 4,624,838 discloses a process for removing acid gases from a gaseous stream by contacting the stream with an aqueous scrubbing solution containing a hetero nitrogen compound comprising either a five- or six- membered ring having a pKa no greater than about 8. Preferred hetero nitrogen compounds include imidazole and piperazine compounds.
U.S. Pat. No. 5,128,049 discloses a method for reducing the hydrogen sulfide content of hydrocarbon-containing fluids and aqueous solutions by injections of a dilute solution of a scavenging agent. Suitable scavenging agents include hexahydro-1,3,5-tris(2-hydroxyethyl)-S-triazine and various other compounds.
There is a long-felt need in the art for a method for regenerating sulfur scavenging compounds. Regenerating such compounds is not only environmentally desirable but is cost efficient and may reduce or eliminate the need for expensive processing equipment. In addition, since the scavenging compounds are regenerated, the need for purchasing replacement scavenging compound is reduced or eliminated.